The relevant state of the art in this field includes processes in which coal and coke are used as suppliers of energy, with oxygen as oxidant, to melt down scrap. A very advantageous process of this kind is described in German Patent No. 28 38 983.
In this known process carbonaceous fuels and oxygen are introduced beneath the surface of the melt bath while additional oxygen is blown as a free jet on to the surface of the bath so that the reaction gases are after-burned and a substantial part of the heat thus generated is transferred to the bath. The best results are obtained in this process when about 70% of the amount of oxygen required is blown on to the bath and the rest of the oxygen enters the melt with the coal beneath the surface of the bath.
Under these conditions up to about 30% after-burning is achieved with an efficiency of retransfer of the heat of combustion to the bath of about 80%. The remaining 20% of the heat of combustion remains in the exhaust gas and accordingly leads to an increase in the temperature of the exhaust gas.
In using the process it has further been found that an increase in the degree of after-burning generally leads to a decrease in the efficiency of retransfer of the heat generated to the bath. In practice about 20% lower heat transfer means an increase of about 200.degree. C. in the exhaust gas temperature. Thus under the given conditions there is a practical limit to the use of the process at about 30% after-burning of the exhaust gases, since further increase in the after-burning impairs the service life of the refractory lining too much owing to the increased temperature stress. Even so the known process has considerable advantages, since for example, using coke as fuel and about 30% after-burning, about 1.5 times as much heat is available as in a process without after-burning.